As is generally well known in the railway engineering arts, brakes on a train are generally controlled by a brake line which carries compressed air from the locomotive of the train to braking systems in the individual railroad vehicles of the train. It is also well known that the classical airbrake system as derived from the Westinghouse airbrake maintains compressed air in the brake line when brakes are not required, and signals a need for brake application by dumping air from the brake pipe in the locomotive to decrease the pressure in the brake air line when a brake application is called for. This system has the desirable feature that a failure of the brake air line causes application of brakes throughout the train.
Unfortunately, the time needed for a pressure decrement to propagate down the line of railroad vehicles in a train can be quite large, a minute or more for a mile-long freight train. Hence, brakes on cars remote from the engine do not help in is stopping the train until some time has elapsed following the brake application.
More rapid methods are known for transmitting a signal for brake application down the length of a train, which, for example, include the use of electrical wires, electromagnetic signals, or optical transmission. For railroad braking systems, the classical brake air line may be combined with radio transmission, particularly in a train having locomotives distributed at various locations along the train.
The WABCO EPIC.RTM. brake system combined with a radio communication link from Harris LOCOTROL.RTM. provides a system in which a brake application signaled by the lead locomotive of a train is accompanied by a radio signal sent from the lead locomotive to slave locomotives in portions of the train remote from the lead locomotive. As usual, with railroad airbrake systems, the lead locomotive dumps brakeline air, which sends a pressure decrement down the line of cars, causing a brake application as it proceeds. In addition, the radio signal is immediately received in locomotives remote from the lead locomotive, and these also begin venting brakeline air. Brakeline pressure decrements then begin to travel along the succession of railroad vehicles from each slave locomotive, causing the brakes to be applied as the pressure decrement reaches each vehicle.
Operation of this system requires, in each locomotive which supplies air to the brakeline, a measurement of the flowrate of air from a main air reservoir in the locomotive to the brakeline of the locomotive. The air pressure in the main air reservoir is maintained by a compressor in the locomotive. This flowrate can be used for a number of purposes. One thing it is used for is to determine the leakage flowrate. This is the flowrate of air which leaks out of the brakeline anywhere in the train. An accurate value for this flowrate is also desirable when the train is being prepared for travel, or after a brake application. In both of these cases, the brakeline pressure must be brought up to the operating pressure value. By measuring the flowrate to the brakeline, the system can determine when the bakeline is charged. This occurs when the measured flow through the orifice is approximately equal to the leakage flowrate. A signal indicating the flowrate is also sent to an alarm system, which interprets a sudden increase in flowrate as indicating a failure of the brake air line. This causes brake application, and a signal to the lead locomotive to stop the train.
The flowrate of air is measured by an orifice, such as the air path constriction in the L19 flowblock, which is located between the main air reservoir, and the brakeline.
Another difficulty with the prior art systems is due to the following considerations: One is that during steady state, when the brakes are not being applied, the brakeline continuously leaks air, and demands air from the main reservoir. Hence, the main reservoir loses pressure continuously through the orifice supplying the brakeline, and is resupplied with air by pulses of air originating in the compressor. A brake control valve placed downstream of the orifice, and upstream of the brakepipe, controls the pressure downstream of the valve in the brakepipe. The brake control valve controls airflow by a mechanical linkage to a diaphragm in an equalizing chamber. On one side of the diaphragm, in the equalizing chamber space, pressure is maintained at a constant value by pulses of air from the main chamber, and pulses of air discharged from the reservoir to the atmosphere by means of electric magnet valves. The other side of the diaphragm in the equalizing chamber has the pressure of the brakepipe. Motions of the diaphragm cause changes in the opening provided through the main control valve, to maintain the pressure in the brakepipe at a predetermined value. An unfortunate aspect of this system is that since the pressure in the equalizing space is increased or decreased in pulses, the diaphragm between the equalizing space and the space connected a pressure port on the brakepipe is moved in pulses. This directly causes pulsed variations of flow through the main control valve, and hence through the orifice which is used to measure the flow through the main control valve to the brakepipe. The signal indicating brakepipe flow, therefore, has pulsed variations due to the pulses of air admitted to or exhausted from the equalizing space.
Furthermore, the brakepipe generally has longitudinal waves caused, for example, by changes in acceleration of the train. When these waves encounter the main control valve, they change the pressure of air in the space next to the diaphragm on the side of the diaphragm toward the brakepipe. This causes the diaphragm to move, which causes a pulsed change in the flow through the main brakevalve. This flow is directly read by the pressure transducer or transducers which measure the pressure drop across the orifice, which is immediately upstream of the main brake valve. Hence, a change of flow is indicated. This tends to set off alarms which are intended to signal the situation of a severe leak or break in the brake line.
These pulsed variations have a particularly undesirable effect when these signals are differentiated in time to detect changes in flow to the brakepipe, as is necessary for responding quickly to a failed brakepipe. The differentiated flow signal may cause false indications of a failed brakepipe situation.